I have been an avid hub gear user for more years than I care to recall. This has usually necessitated becoming acquainted with the internals at some point with many of the hubs that I have used.

I recently obtained a SRAM A2 Automatix hub (thanks Alan!) for my urban bike build (for which there is presently no technical manual on the SRAM website) and curiosity got the better of me; here are the pictures.

very easy to ge this far if you have a 22mm cone spanner

the centre; almost everything can be inspected without further disassmbly

The bearing adjustment is via locknuts on the LHS. These are 22mm AF; you need one thin one (about 3mm thickness) at least when the hub is out of the frame. Fortunately it turned out that I had already made a flat spanner for an angle grinder I use, so it came in handy here.

The brake plate carries the LHS cone for a large diameter bearing comprising 12off 3/16" balls in a clip. The brake plate is a sliding fit on a ground diameter. *Important* -adjusting this bearing is like adjusting a conventional headset; tightening the locknuts together also tightens the bearing adjustment. It is best to nip up the locknuts with the bearing loose, then experiment with tighter settings until the play just disappears when the locknuts are tight. Taking the free play out with the inner locknut, then tightening the second one will produce an enormous bearing preload which will quickly ruin the hub.

Removal of the LHS locknuts allows the hub centre to come out in one piece as shown. As supplied there was light oil on the mechanism and grease on the main bearings and pawls. Probably not enough of either in fact; I'd suggest a year's use would be more than enough with this amount of lubricant inside.

The drag spring controls the brake actuation and hardly creates any drag even when new. I expect this drag to reduce as time goes on.

The shift control spring holds the centrifugal weights inwards until the hub is turning above a certain speed, at which point the pawl control plate rotates forwards slightly which allows the high gear pawls to engage with the high gear drive ring. The high gear drive ring is in constant mesh with the hub body via external dogs. The high gear drive ring is fitted with the only non-metallic internal part I could see which is a rubber 'O' ring. I believe this is simply to prevent the drive ring from rattling slightly when the hub is in low gear.

The right side cone is fixed adjustment; it is held by the right side locknut up against a shoulder on the axle. This means the right side locknut must be snug at all times in use. If the right side locknut and cone are removed, this allows some inspection of the RHS bearing surfaces/laybrinth seal, and also the planet gears themselves are just visible beyond the bearings. Since removal and replacement of the RHS cone cannot disturb the bearing adjustment, this is probably a convenient access point to allow further lubricant to be added to the hub. The axle can be held on its flats in a bench vice, and then a 17mm spanner is all that is required to remove the RHS locknut and cone; it could hardly be any easier.

The hub has heavy serrated washers (not shown) in place of dogged 'anti-turn' washers. Because the net torque on the axle is one way, and not that great either, maybe this is a good solution. It certainly avoids the possible need for different anti-turn washers for different frames. I will see how I get on with this arrangment, and be ready to fit a conventional anti-turn washer if needs be.

The low gear is 1:1 so a largeish rear sprocket is required. The hub came with a 19T cog which takes the usual hub gear fitting with three lugs. I think I may go with 42/23 gearing if I can find a 23T sprocket. This will give a 137% high gear in the high 60's on a 27" wheel.

The brake comes on with a 45 degree rotation of the sprocket carrier at the hub. This means with a ~2:1 drive ratio a 1/16th turn of the cranks backwards. Presumably this will increase slightly should the brake shoes wear.

The hub has shields at each end which should prevent most rubbish from getting in. However these are not full contact seals by any means (and maybe they couldn't ever be bearing in mind the heat that the brake generates); it is quite possible that water could enter the hub at either end (it is about as well sealed as a SA three-speed if you like). The structure of the hub is such that in use, both water and any fluid oil will not stay in the hub; there are no large shoulders or lips inside the hub to retain oil.

Full grease lubrication would risk that the shift control mechanism will be sluggish in cold weather.Oil lubrication will probably work OK but would need to be very regular to retain an adequate amount within.

I added grease to the bearings and to the planet gears (there was hardly any on them that I could see) and used an aerosol fluid grease (meant for chains) on the rest of the mechanism before reassembly. A decent smear of grease behind the shields may help to deter moisture ingress.

I may experiment with using a semi-fluid grease inside this hub; occasional shots behind the RHS cone will keep the planet gears sweet and will gradually migrate through the rest of the hub; if necessary purging with oil from time to time may be a good idea too. Some net loss of lubricant from the LHS may be no bad thing; it should carry away any wear debris from the coaster brake . I may fit an oiler to the hub shell at some point in the future.

If you are a 'fit and forget' merchant then I'd suggest a yearly wash and brush-up for the centre; as depicted it comes out in one piece and (apart from the ball rings and brake shoes) nothing else will fall off the assembly. This means it can be washed out in solvent, regreased and reinstalled. I anticipate the main threats to the hub longevity may be;

1) water ingress causing corrosion of the shift control spring or binding of the mechanism. If the shift control spring fails the gear will default to the high gear. If the mechanism binds it could stick in either.

2) wear of the planet gears. There wasn't an awful lot of grease on mine and this could be a problem after a few thousand miles. I intend to set the bike up so that most riding is in the high gear (which uses the planet gears) so they will see a lot of use.

3) ball clip failure; if either ball clips break up the hub will quickly fill with shrapnel and cause collateral damage. I may experiment with loose balls (although this may make hub reassembly either very difficult or even impossible).

The low gear pawls are about the same size as those inside a cassette freehub, so should be strong enough for most people. the high gear pawls are larger still. If either fails the other gear should still work OK so total drive loss is unlikely. If the shift control spring or drag spring fail and spare parts are not available, I think suitable replacements could be made quite easily using piano wires of the correct gauge. Note that the shift control spring force controls the shift point; it would have to be exactly correct.

In the event of complete hub failure, fitting a new centre is a ten-minute job, and the only moderately likely thing that might stop this as a repair route is the possibility of wear on the 'ball cups' which are integrated into the hubshell.

Overall it is a rather workmanlike piece of kit; my only minor criticisms of it thus far are concerning the sealing and the rather small amount of OE grease inside the unit.

I hope this is of interest to others; obviously I've used my many years of experience to come up with the above comments and suggestions, but the usual disclaimer applies; none of it is sanctioned by SRAM, you carry out any of it at your own risk, it may void the warranty... etc etc...

---------------------------------------update nov 2014.

I've dismantled a few of these hubs now and my fears concerning water ingress appear to be well-founded, with signs of corrosion on unmaintained units (i.e. ones that have been used with the factory lube only) obvious after very little use in UK weather.

On the plus side SRAM have finally deigned to put some technical information on their website;

but sadly, all it shows is basically that you can buy the external parts, or, erm, a new hub. So not at all helpful really. No internal spare parts, no part numbers that you can search for.

They appear to supply OEM hubs in at least two forms, with different shift control springs. AFAICT you can't buy either spring as a spare part. I am working on the manufacture of a revised shift control spring to give more choice of shift point. Actually swapping the spring out isn't that difficult, it is just a bit fiddly. Once the large 'E' clip is removed from the left side of the internal, the low gear/brake assy comes out and then the shift control/high gear pawl/ring gear unit comes out in one lump; removal of a second (much smaller) 'E' clip then allows one of the pawls and the shift control spring to be removed. The 'E' clip may need a slight squeeze so that it is snug when refitted, but the main risk here is that you lose the 'E' clip; it really is tiny!

BTW for those that like counting teeth, the ring gear is 47T, the sun is 17T and the three planets are 15T. This gives a high gear ratio of 1.3617 (I think). I believe this also means that the planet pinions are not evenly spaced in the planet cage, and that the planet cage will (inevitably, with wear) start to orbit slightly eccentrically. Whether this is a real concern that will cause operational troubles remains to be seen. The use of prime number tooth counts on the ring and sun is presumably meant to help prevent noises and certain wear patterns.

I import small batches of the SRAM Automatix hubs from the US as there isn't any official distributor in the UK AFAIK.They can be supplied as a bare hub kit, or built into a wheel. If you want any further details and prices either PM me, or email to just1bike[at]gmail.com

it is just a question of giving the shift control spring a different 'set', in principle...

It isn't a very big job to get as far as the spring, but if you are not familiar with hub gears it will take a bit longer and the risks will be a bit higher too. Do bear in mind that spare parts are not readily available that I know of....

Personally I'd be tempted to make a new spring from piano wire, and mess about with that. Otherwise there is a danger that the original spring is made unusable and then you are stuck.

If people are keen I can think about making some new springs that you can tweak to suit yourself.

I have a SRAM A2 hub installed on my commuter bike. I have the freewheel version. I've noticed a rattle in the hub since new, and want to make sure it isn't a defect or problem with the hub. I hear the hub when coasting over bumpy surfaces. When pedaling, while the hub is engaged, I don't hear the rattle. It's not a loud rattle but I do notice it. I used to have a Shimano Nexus 3 on this bike, and it didn't make the rattling noise.

To me, it sounds like the rattle involves either the drive ring or possibly something with the centrifugal weights. I don't have a 22mm flat wrench for the inner LHS locknut, so I haven't taken the hub apart.

P.S. Anybody know where a 22mm flat wrench can be purchased in the US?

scooby214 wrote:To me, it sounds like the rattle involves either the drive ring or possibly something with the centrifugal weights. I don't have a 22mm flat wrench for the inner LHS locknut, so I haven't taken the hub apart.

P.S. Anybody know where a 22mm flat wrench can be purchased in the US?

22mm (or 7/8") flat wrenches are needed to remove wheels and other tools from many angle grinders and routers; I'd suggest that a trip to a good tool store might be useful.

The rattle could well be the centrifugal weights inside, or the pawl control plate. Note also that the high gear drive ring is set on an 'O' ring cushion to prevent such rattles; if yours is missing then this would explain it.

If you are pedalling at speed, but not quite 'keeping up' with the bike, so transmitting no force through the pedals, does the rattle go away? If it does then I would say that this is a clear sign it is the centrifugal weight mechanism.

Brucey wrote:22mm (or 7/8") flat wrenches are needed to remove wheels and other tools from many angle grinders and routers; I'd suggest that a trip to a good tool store might be useful.

The rattle could well be the centrifugal weights inside, or the pawl control plate. Note also that the high gear drive ring is set on an 'O' ring cushion to prevent such rattles; if yours is missing then this would explain it.

If you are pedalling at speed, but not quite 'keeping up' with the bike, so transmitting no force through the pedals, does the rattle go away? If it does then I would say that this is a clear sign it is the centrifugal weight mechanism.

Maybe some fresh grease might make it quieter?

cheers

Thanks for your response. I found a source for a 22mm flat wrench, through Universal Cycles. I already had some things to order, and should have the wrench by the end of the week. I'll check my hub out when I get the wrench. I can say that there is a small amount of play in the bearings. I can feel the play when moving the rim side-to-side. I don't think that is causing the noise, but I can do a better job of adjusting the bearings without preloading them or having play.

I got my 22mm cone wrench today. I removed the internals from the hub shell. I found nothing obviously wrong, other than there being very little lubrication on the bearings. I had noticed play in the bearings before taking the hub apart, but didn't see any damage to the bearings or bearing races. The centrifugal weights were slightly loose, but moved freely and didn't appear to be at fault, so I didn't do anything with them. The o-ring cushion was in place and didn't appear to have any problems.

I greased the bearings, put a little bit of light synthetic grease on the low gear pawls, and put the hub back together. I was careful to get the play out of the bearings without overtightening them. The result is a hub that is quieter than before. I can still hear the noise, but it's much quieter now. The hub shifts at the same point, but the high gear ticking noise is very quiet now due to lubing the pawls.

This is a very simple internal gear hub. It's a shame SRAM doesn't provide repair parts for the hub, as I think it would be easy to repair a damaged A2, as opposed to replacing the entire hub internals.